A Manual Of Photography

PRECIPITATES OF SILVER SALTS. 101 constantly abstracting matter from its surface in place of intro ducing new. „ “Exposed in this state to the focus of a camera with the glass towards the incident light, it became impressed with a remark- ably well-defined negative picture, which was direct, or reversed, according as looked at from the front or the back.. On pouring over this cautiously, by means of a pipette, a solution of hypo- sulphite of soda, the picture disappeared, but this was only while wet; for on washing in pure water and drying, it was restored, and assumed much the air of a daguerreotype when laid on a black ground, and still more so when smoked at the back, tue silvered portions reflecting most light, so that its characters had, in fact, changed from negative to positive. From such a pic- ture (of course before smoking) I have found it practicable to take photographic copies ; and although I did not, in fact, suc- ceed in attempting to thicken the film of silver by connecting it, under a weak solution of that metal, with the reducing pole of a voltaic pile, the attempt afforded distinct indications of its practicability with patience and perseverance, as here and there over some small portions of the surface, the hg is 1 * -b a full metallic brilliancy under this process. I would oidy U- tion further, to those who may think this experiment worth repeating, that all my attempts to secure a good result by dry- ing the nitrate in the film of chloride have failed, the crystallisa- tion of the salt disturbing the uniformity of the coating. 10 obtain delicate pictures the plate must be exposed wet, and when withdrawn must immediately be plunged into water. lhe nitrate being thus abstracted, the plate may then be dried, n which state it is half fixed, and it is then ready for the hypo- sulphite. Such details of manipulation may appear minute, but they cannot be dispensed with in practice, and cost a great dea of time and trouble to discover.. " This mode of coating glass with films of precipitated arg tine or other compounds, affords, it may be observed, the on y effectual means of studying their habitudes on exposure to igi, free from the powerful and ever-varying influence of the size in paper, and other materials used in its manufacture, and estimat- ing their degree of sensibility and other particulars of then deportment under the influence of reagents. 1 find, for example, that class so coated with the iodide of silver is much more sen- süve than ff similarly covered with the chloride, and that if both bo washed with one and the same so Intion of nitrate, there is no comparison in respect of this valuable quality ; the iodide being far superior, and of course to he adopted in preference, for the use of the camera. It is, however, more difficult to hx, the